Water bond enthalpy

Compound Molar mass, g-mol 1 Melting point, °C Boiling point, °C pK in water Bond enthalpy, kJ mol 1 Bond length, pm... 

The most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

To rationalize the effect of polar groups on and Sj, we can imagine that polar interactions with the water molecules around the solute cavity replace some of the hydrogen bonds between the water molecules. As indicated by the experimental data, this loss of water water interaction enthalpy seems to be compensated by the enthalpy gained from the organic solute water polar interactions. At this point it should also be mentioned that additional polarization effects could enhance the interaction between the organic solute and the water molecules in the hydration shell... 

An Arrhenius-type analysis of temperature dependence can be used to calculate the enthalpy and entropy of activation for the relaxation process. For liquid water, the enthalpy of activation is 19 kjmol-1, which corresponds approximately to the energy required to break one hydrogen bond. For ice, the equivalent enthalpy is 54 kj mol-1,... 

It can be added that the shifts are of the order of H bond enthalpies (1-7 kcal per H bond corresponds to 350-2500 cm shift), and that the shift in water is often about twice as large as in methanol. 

Also, Schellman s work is pertinent (1809). From studies on heats of dilution of urea in water he concludes that the N—H 0=C bond has an enthalpy of 1.5 kcal/mole in aqueous solution, and he carries this value over to proteins and polypeptides. Among these complicated materials he is forced to approximate—but he deduces relations which show the stability of helices and sheets in terms of H bond enthalpy and configurational entropy. From this he draws the important conclusion that H bonds, taken by themselves, give a marginal stability to ordered structures which may be enhanced or disrupted by the interactions of the side chains. Schellman ends his papers with a discussion of experimental tests needed to eliminate some of the assumptions in his theoretical analysis. 

A colorless, odorless, flammable gas that is nearly insoluble in water and highly toxic Has an extremely large bond enthalpy (1074 kJ-mol" ). 

The mean bond enthalpy for the stepwise dissociation of both O-H bonds in water is (499 + 428)72 = 463.5 kjmol-h... 

This data does not mean that the O-H bonds in water are different. On the contrary, they are identical in every respect. The different values of bond enthalpies for the O-H bonds arise because they apply to very different species, H2O and... 

Thus, not only is the dissociation of the oxygen-hydrogen bond rare, but only a fraction of the ions generated escape recombination. These dynamic processes need to be consistent with the low concentration of protons in pure water, which in turn is determined by the competition between the bond enthalpy of an 0-H bond and the solvation energy of the hydroxide and hydronium ions. 

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